It is generally desirable that components used in automotive and aerospace applications be made as light as possible. Lighter components contribute to lowering the overall weight of an automobile or aircraft and so assist in improving fuel economy. The use of lightweight components may also provide other advantages such as, in automotive applications, improved handling performance, and, in aerospace applications, allowing a heavier load to be carried. For these reasons, it is desirable to make components for such applications from lightweight alloys, such as aluminium alloys (Al-alloys).
Al-alloys are, however, less ductile than, for example, steel alloys. As a result, it is at least difficult, and sometimes not possible, to form components of complex shape from Al-alloys. Instead, components of complex shape are sometimes milled from solid blocks of heat treated Al-alloy. This can result in a high percentage of the Al-alloy being wasted, and hence in high costs of manufacture. The same is true when forming components from magnesium alloys (Mg-alloys).
WO 2008/059242 discloses a method of forming aluminium alloy (Al-alloy) sheet into components of complex shape. The method disclosed in WO 2008/059242 includes the following general steps:
(i) heating an Al-alloy sheet blank to its solution heat treatment (SHT) temperature and maintaining that temperature until SHT has been completed;
(ii) rapidly transferring the sheet blank to a set of cold dies so that heat loss from the sheet blank is minimised;
(iii) immediately closing the cold dies to form the sheet blank into the component; and
(iv) holding the formed component in the closed dies during cooling of the formed component.
Whilst this method has certain advantages over earlier methods, it also has certain drawbacks. For example, the forming needs to be carried out before the sheet cools in order for the method to be successful. As the sheet tends to cool quickly (it is thin and has a low specific heat capacity and high thermal conductivity), the forming must be carried out very quickly. This is problematic in that the forming therefore requires a very quick press with high forming forces. Such presses are expensive and high forming forces tend to shorten tool life. Also, it is difficult to form complex parts: the sheet tends to cool before the complex part can be fully formed.
It is therefore desirable to address this drawback.